Corundum is a crystalline form of aluminium oxide containing traces of iron, titanium and chromium. It is a rock-forming mineral, it is a transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. Corundum has two primary gem varieties: sapphire. Rubies are red due to the presence of chromium, sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange; the name "corundum" is derived from the Tamil-Dravidian word kurundam. Because of corundum's hardness, it can scratch every other mineral, it is used as an abrasive on everything from sandpaper to large tools used in machining metals and wood. Some emery is a mix of corundum and other substances, the mix is less abrasive, with an average Mohs hardness of 8.0. In addition to its hardness, corundum has a density of 4.02 g/cm3, unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.

In 1837, Marc Antoine Gaudin made the first synthetic rubies by reacting alumina at a high temperature with a small amount of chromium as a pigment. In 1847, Ebelmen made white synthetic sapphires by reacting alumina in boric acid. In 1877 Frenic and Freil made crystal corundum from. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF2 and Al2O3 with a little chromium at temperatures above 2,000 °C. In 1903, Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process; the Verneuil process allows the production of flawless single-crystal sapphire and ruby gems of much larger size than found in nature. It is possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis; because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market causing a significant reduction of price in recent years. Apart from ornamental uses, synthetic corundum is used to produce mechanical parts, scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft, laser components.

For example, the KAGRA gravitational wave detector's main mirrors are 23 kg sapphires, Advanced LIGO considered 40 kg sapphire mirrors. Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a= 4.75 Å and c= 12.982 Å at standard conditions. The unit cell contains six formula units; the toughness of corundum is sensitive to surface crystallographic orientation. It may be 6–7 MPa·m​1⁄2 for synthetic crystals, around 4 MPa·m​1⁄2 for natural. In the lattice of corundum, the oxygen atoms form a distorted hexagonal close packing, in which two-thirds of the gaps between the octahedra are occupied by aluminium ions